Am I doing it right???

yjmudder90 · #1 (**permalink**) 03-04-2008, 10:02 PM

When i drive my Jeep i usually drive above 1000 rpm and below 2500 rpm. is this bad for my engine/gas mileage?? should i go higher??? it just sounds like it is revving way high above 2500 rpm. I get about 11 mpg with stock (3.07) gearing and 33" tires. I have heard of people revving to 3000 and higher but it doesnt sound healthy to me. any thoughts or input are highly appreciated.

TerryMason · #2 (**permalink**) 03-04-2008, 10:47 PM

My 4.2 would not rev above 2500 until I did the TFI ignition upgrade. After that It would rev easier, and sound better.

Is your idle at 1000? That may be a little high, but I dont' think driving between 1000 and 2500 will harm anything.

yjmudder90 · #3 (**permalink**) 03-04-2008, 10:51 PM

when my Jeep is cold it idles at about 650 but after it warms up it idles at 1000. it will go above 2500 but i dont know about the TFI ignition. i think my ignition is just stock. thanx for the input though.

yjmudder90 · #4 (**permalink**) 03-04-2008, 10:52 PM

should i be revving higher???

jay79cj7 · #5 (**permalink**) 03-05-2008, 12:43 AM

Higher revs will generally burn more gas. (speed being the same)

jonsolbe · #6 (**permalink**) 03-05-2008, 03:16 AM

If it sounds like you're reving too high, then you probably are. If your's is a 4.0 inline 6, it's a high torque, low rpm engine. Most of it's power comes from a lower power curve, as opposed to a small cylinder, high reving 4 cylinder engine like in a small import sports car or sport bike. It's akin to reving a V-twin motorcycle engine to high RPM, they're not designed for it. Also, most vehicles are designed to idle a bit faster when cold, and then slow down when they warm up. If yours is idleing faster when warm, it sounds like there may be a problem. Vaccum leak?

TerryMason · #7 (**permalink**) 03-05-2008, 07:32 AM

Assuming my gear ratio calculator (Gear Ratios)is still correct, you should be doing 2039 rpm when you are going 65 mph, so I think nothing is "wrong" with your setup.

Look into doing that TFI, or another ignition upgrade - it will really help the feel of your Jeep. The stock ignition on those 4.2L engines are pretty bad.

yjmudder90 · #8 (**permalink**) 03-05-2008, 07:40 AM

yeah i am going to do the tfi ignition this week. thanks for the info.

southtj · #9 (**permalink**) 03-05-2008, 07:42 AM

The idea is to keep rpm as low as possible while driving. The lower the revs the lower the gas consumption. There is nothing wrong with going over 2500 before shifting but you probably shouldn't keep it there for long periods of driving.

jfrabat · #10 (**permalink**) 03-05-2008, 11:54 AM

Quote:

Originally Posted by jay79cj7

Higher revs will generally burn more gas. (speed being the same)

I think it depends on the load placed on the engine more than the RPMs. If you are driving at 30 mph, but have to floor it to increase speed slightly, you may be better off just donwshifting to accelerate (less load on the engine). I think so, anyway!

Megamole · #11 (**permalink**) 03-07-2008, 12:20 AM

I am supprised you are having that good of milage from those 33" and 3.07 gears.

When you check your milage have you taken into account that your odometer is off by 14%. I used a tire calculator http://www.dakota-truck.net/CGI-BIN/TireCalc.cgi and after driving a real mile in your Jeep your odometer will only show you have gone .86 miles.

I have basically the same jeep minus the tire size. I have 31"ers I have done the TFI upgrade, have a weber carb, Juice Box ignition and a Borla exhuast. My Jeep will get 14-15mpg. My milage is also off 1 mile equals .91 miles driven. When I calculate my mpg, I add on 10% to the milage, then divide. This gives me a more accurate mpg.